A standard, fully differential, operational amplifier has specific requirements as regards the bias voltage which must be supplied thereto, in order for it to operate effectively. Specifically, the amplifier usually includes at its input a single pair of differential transistors. If PMOS input devices are used, then, in order to provide suitable bias voltages to these transistors at low supply voltages, the input reference voltage level needs to be nearer to the negative supply voltage than to the positive supply. This restricts the voltage swing which can be handled by the input stage, and hence restricts the maximum available dynamic range at the output of the operational amplifier. Likewise, if NMOS input devices are used, the input reference voltage level needs to be nearer to the positive supply voltage than to the negative supply.
U.S. Pat. No. 5,565,813 discloses a low voltage differential amplifier, in which the voltage on a capacitance circuit, which is switched in and out of the circuit, is used to provide a bias voltage.